In the coming months Mozilla will launch support for an open ecosystem of extensions on Firefox for Android on addons.mozilla.org (AMO). We’ll announce a definite ...
It was until FF68, when Mozilla decided that it would improve their market share if they rewrote the whole browser and kicked all their advantages down the drain.
Yes, if you run beta/nightly, create an account on their system, and add the extension to a collection there. That's a lot of hoops to jump through, and it reduces the possible audience for an extension developer to a tiny fraction, so nobody has put any effort into building extensions for Firefox on Android.